Magnetic and crystallographic properties of molecular beam epitaxially grown Fe3O4/NiO superlattices and Fe3O4 films

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Abstract
Ferromagnetic resonance, SQUID magnetometry, and x‐ray diffraction have been used to characterize a set of [Fe3O4(68 Å)/NiO(17 Å)]N superlattices (SL) with N=3, 10, 30, and 100, as well as a 1.5‐μm‐thick Fe3O4 film. For this NiO thickness, Fe3O4 layers are strongly coupled and the in‐plane anisotropy is much less than the 330‐Oe ferromagnetic resonance (FMR) linewidth at 35 GHz. Both in‐plane and perpendicular FMR at 9.5 and 35 GHz have been used, with the 9.5‐GHz data showing significant hysteresis associated with the sample magnetization. X‐ray diffraction indicates that both the film and SL’s are nearly cubic single‐crystalline structures with long‐range coherence. The 300 K magnetization data indicate the presence of small cubic anisotropy in the SL’s, although bulklike Fe3O4 magnetic ordering in the thick single film. When the Fe3O4 film is cooled below the Verwey transition in a 10 kOe field (aligned along 〈100〉), the FMR shows that the sample develops a large uniaxial (Ku=1.8 kOe) in‐plane anisotropy with its easy axis along the field direction. Decreased microwave loss also occurs abruptly at the film TV.